Last name of father and mother, respectively.

  let motherLastName = "Carter Hughes"
  let fatherLastName = "Miller Thompson"
  let childLastName  = "Miller Carter"
  let childFullName  = "Jean Paul Miller Carter"

Or so that is how it works in many countries around the world.

You might ask, —“Why does the father’s last name go first and the mother’s second?”— That’s an old tradition, and it can change whenever enough people in our society agree. As it stands, the father’s family name tends to persist down the family tree, while the mother’s family name often disappears in each generation.

Or so that is how it works in many countries around the world.

You should have given a more complete example, where the parents themselves have long names to demonstrate that something does have to get dropped when you have children.

Hughes and Thompson were both dropped in their example.

My bad, I misread the parents' names as their full names.

Ok. So your children get their grandfather's names.

The names of their grandmothers get dropped.

Only a partial improvement over just dropping the mother's name.

> What last name do your kids use?

In the country where he lives (Belgium), the parents get to decide which family name the kids get.

But I think you only get to decide for the first kid? All following kids will have the same family name. At least that's the rule in the Netherlands.